Motor speed estimation for drive safety system

ABSTRACT

According to an aspect, there is provided a method for evaluating safety of a speed of a motor controlled by a frequency converter. The method includes, first, measuring first, second and third phase currents of three-phase electric power fed from the frequency converter to the motor and forming first, second and third current measurement pairs based thereon. Then, the speed of the motor is estimated separately based on each measurement pair to produce respective first, second and third estimates for the speed of the motor. A voting logic is applied to the first, second and third estimates for the speed of the motor. A voting logic is applied to the first, second and third estimates. An output of the voting logic is fed to a safety logic controlling at least one safety function of the frequency converter. The output of the voting logic includes an estimate for the speed of the motor and an indication whether or not said final estimate is valid according to the voting logic.

TECHNICAL FIELD

Various example embodiments relate to control of industrial processes.

BACKGROUND

Drives (or equally frequency converters) are used to control the motionof machines, typically to achieve optimal performance and efficiencyfrom the given machine or machines. Drives are employed in manyapplications that require precise motion control, for example, in lineautomation applications employing lifts, cranes and/or conveyor belts.In many applications, it is often of paramount importance that the speedof the motor does not exceed certain pre-defined limits allowed for safeoperation. Thus, various schemes for estimating safe motor speeds havebeen implemented to ensure that this does not happen. Many of suchschemes are based on monitoring voltage or two or three phase currentassociated with the motor. However, said established schemes oftenstruggle or even fail altogether when they are applied to low speedapplications where deriving useful voltage/current measurements is oftendifficult.

Therefore, there is a need for a better way for safety monitoring ofmotor speeds so as to overcome or alleviate at least some of theaforementioned problems.

BRIEF DESCRIPTION

According to an aspect, there is provided the subject matter of theindependent claims. Embodiments are defined in the dependent claims.

One or more examples of implementations are set forth in more detail inthe accompanying drawings and the description below. Other features willbe apparent from the description and drawings, and from the claims.

Some embodiments provide a method, an apparatus, a system and computerreadable media for evaluating speed or frequency of a motor controlledby a frequency converter.

BRIEF DESCRIPTION OF DRAWINGS

In the following, example embodiments will be described in greaterdetail with reference to the attached drawings, in which

FIG. 1 illustrate an exemplary industrial system according toembodiments;

FIGS. 2A and 2B illustrate exemplary voting architectures according toembodiments;

FIG. 3 illustrates a process carried out by a safety logic according toembodiments; and

FIGS. 4 and 5 illustrate apparatuses according to embodiments.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The following embodiments are only presented as examples. Although thespecification may refer to “an”, “one”, or “some” embodiment(s) and/orexample(s) in several locations of the text, this does not necessarilymean that each reference is made to the same embodiment(s) orexample(s), or that a particular feature only applies to a singleembodiment and/or example. Single features of different embodimentsand/or examples may also be combined to provide other embodiments and/orexamples.

A general architecture of a system to which embodiments of the inventionmay be applied is illustrated in FIG. 1. FIG. 1 illustrates a simplifiedsystem architecture only showing some elements and functional entities(namely, showing only some functional safety related elements andfunctional entities), all being logical units whose implementation maydiffer from what is shown. The connections shown in FIG. 1 are logicalconnections; the actual physical connections may be different. It isapparent to a person skilled in the art that the system may alsocomprise other functions and structures.

FIG. 1 illustrates a system comprising a drive 100 controlling a motor111. The drive 100 (equally called a motor drive) comprises at least avoting-based speed analysis unit 108, a safety logic 109 and a STOsafety function 110. The motor 111 may be further connected to amechanical system so that the drive 100, the motor 111 and themechanical system form together an industrial system (e.g., a productionor assembly line system or a part thereof).

The drive 100 is a device used for controlling the motion of the motor111. Said control may be achieved by changing one or more driveparameters of the drive 100 which may comprise parameters such astorque, speed, power, voltage, frequency, motor control mode (e.g.,scalar, vector or direct torque control),proportional-integral-derivative (PID) controller settings, accelerationramp settings, deceleration ramp settings and/or other parametersaffecting the operation of the drive. The drive 100 may specifically bean electrical drive (an AC drive supporting low to high voltages and/orlow to high motor speeds). The drive 100 may be equally called afrequency converter. The drive 100 may be a programmable logiccontroller (PLC) or a (motor) soft starter. In an embodiment, the drive100 may be a variable speed drive (VSD) or a variable frequency drive(VFD). Contrary to some definitions of term “drive”, the motor 111 whichis driven does not form a part of the drive 100 itself in the context ofthis application.

The drive 100 may comprise at least one high-voltage power unit (notshown in FIG. 1) for powering the motor 111 and a low-voltage controlunit (comprising at least the elements 108, 109, 110) for providingcontrol signalling to the power unit so as to control the driveparameters of the drive 100 and safety functionalities. In someembodiments, the control and power units may be integrated into a singleunit.

The drive 100 may be connected using a (wired) connection to the motor111 driving industrial or non-industrial processes (i.e., driving amachine, a device, a component, an apparatus or a system for performingan industrial or non-industrial process). Specifically, the drive 100(or specifically its at least one power unit) feeds the motor 111 via athree-phase power supply. The motor 111 may be, for example, anasynchronous motor (e.g., an induction motor), a synchronous motor(e.g., a permanent magnet motor) or a reluctance motor (e.g., asynchronous reluctance motor). The motor may be used for running, forexample, a system for transporting material, such as a pump, a fan, acompressor, a blower, a conveyor belt, a crane and/or an elevator and/ora system for processing materials, such as a paper machine, a mill, astirrer and/or a centrifuge. While FIG. 1 illustrates a single motor111, in other embodiments the drive 100 may be used for controlling anelectrical machine comprising multiple motors.

The voting-based speed analysis unit 108 is used for evaluating speed ofthe motor 111 based on three-phase power fed to the motor 111 and forindicating to the safety logic 109 whether the estimated value of thespeed of the motor is dependable (i.e., whether or not it can betrusted). In addition to or alternative to the speed of the motor 111,the voting-based speed analysis unit 108 may conduct its analysis basedon the frequency of the motor which is a quantity closely connected tothe speed of the motor 111 (as will be discussed in more detail below).The voting-based speed analysis unit 108 according to embodiments willbe discussed in further detail following the discussion of otherelements of the illustrated system.

The safety logic 109 is a unit for managing one or more safety functionsof the drive 100 based on a state of the motor 111 and/or of the drive100. In other words, the safety logic 109 is used for continuouslydetermining whether a particular safety function is to be triggered at agiven time. The one or more safety functions may comprise at least theSTO safety function 110. In addition to the information received fromthe voting-based speed analysis unit 108, the safety logic 109 may beconfigured to receive further information regarding the state of theelectric motor from one or more other elements of the drive 100 and/orfrom one or more elements external to the drive 100. For example, thesafety logic may receive information on the rotation speed of the motor111 from a tachometer. Moreover, the safety logic may employ informationon the (current) drive parameters (as listed above) in its analysis. Thesafety logic may also receive signalling from one or more sensors (e.g.,a presence sensor indicating that a person is presently in a dangerousarea and/or a sensor indicating that a safety related mechanical door orhatch is currently open) and/or an emergency stop button of the drive100.

The STO safety function 110 is a basic foundation for drive-basedfunctional safety used for bringing a drive safely to a no-torque state.The STO safety function 110 may be used for preventing an unexpectedstartup of the motor 111 and/or for stopping the motor 111 (e.g., due tooverheat or overspeed protection purposes or as an emergency stop of themotor 111). Specifically, said emergency stop fulfils the stop category0. The stop category 0 means stopping by immediate removal of power tothe machine actuators (i.e. an uncontrolled stop—stopping of machinemotion by removing electrical power to the machine actuators). Thestopping of the motor 110 may be achieved by shutting down a power unitof the drive 100 in a (pre-defined) controlled manner.

In some embodiments, the drive may comprise (or be connected to), inaddition to the STO safety function 110, one or more other safetyfunctions (not shown in FIG. 1). Said one or more other safety functionsmay comprise, for example, one or more of a safe stop 1 (SS1), a safestop emergency (SSE) 110, a safely-limited speed (SLS), a safe maximumspeed (SMS) and a safe brake control (SBC). Said one or more othersafety functions may also be controlled by the safety logic 109 (and/orone or more further safety logics).

While the safety logic 109 and the STO safety function 110 (and one ormore other safety functions) may be integral parts of the drive 100 (asillustrated in FIG. 1), in other embodiments, one or more of elementsmay form a separate system (external to the drive 100).

As mentioned above, the drive 100 (or the frequency converter) feeds themotor 111 using a three-phase power supply. These three phases aredenoted, in the following, with letters U, V and W following a commonconvention. The voting-based speed analysis unit 108 measures a firstphase current I_(U) 101 of three-phase electric power fed from the drive100 to the motor 111, a second phase current I_(V) 102 of saidthree-phase electric power and a third phase current I_(W) 103 of saidthree-phase electric power. The first, second and third phase currentsI_(U) 101, I_(V) 102 and I_(W) 103 may have phases φ_(U),φ_(V)=φ_(U)+120° and φ_(W)=φ_(U)+240°, respectively, assuming idealoperation. Then, the voting-based speed analysis unit 108 forms first,second and third current measurement pairs, where the first currentmeasurement pair comprises the first and second phase currents (I_(U),I_(V)), the second current measurement pair comprises to the second andthird phase currents (I_(V), I_(W)) and the third current measurementpair comprises to the first and third phase currents (I_(U), I_(W)).

The voting-based speed analysis unit 108 estimates, in elements 104,105, 106, the speed of the motor 111 separately based on each pair ofthe first, second and third current measurement pairs (I_(U), I_(V)),(I_(V), I_(W)), (I_(U), I_(W)) to produce respective first, second andthird estimates n₁, n₂, n₃ for the speed of the motor (that is, therotation speed of the motor). For example, the estimating in elements104, 105, 106 may be carried out so that a separate preliminary estimatefor the speed of the motor based on each current in each currentmeasurement pair is, initially, calculated and, then, each of saidfirst, second and third estimates n₁, n₂, n₃ is calculated based on twopreliminary estimates associated with a corresponding currentmeasurement pair, e.g., as an average.

The speed of the motor 111 may be estimated in any conventional mannersuch as on the basis of the frequency of the current feeding the motor111 (i.e., the frequency of the motor 111), depending on the motor type.For example, the rotation speed of a synchronous motor (synchronousspeed) n (given in revolutions per minute, RPM) may be calculated withthe equation:

${n = \frac{f}{p}},$where f is the frequency of the feeding current in hertz and p is thenumber of pairs of poles of the motor. The rotation speed of aninduction motor, for example, may also be estimated with this equation,although the rotation speed of the induction motor is further affectedby slip Δn. In several applications, however, the effect of the slip maybe ignored and a sufficiently accurate speed estimate may still beobtained.

The frequency of the feeding current (i.e., the frequency of the motor)may be determined based on a current, for example, by using a phasedlocked loop. Alternatively, the frequency of the feeding current may bedetermined based on a zero crossing method, that is, by monitoringspacing (or time period) between consecutive time instances when thefeeding current is zero (or alternatively, when the sign of the feedingcurrent function changes from positive to negative or vice versa).

In some embodiments, the voting-based speed analysis unit 108 mayestimate, in elements 104, 105, 106, instead of the speed of the motor,the frequency of the motor which, as discussed above, is indicative ofthe speed of the motor (being directly proportional to it in mostapplications). In other words, the voting-based speed analysis unit 108may estimate, in elements 104, 105, 106, the frequency of the motor 111separately based on each pair of the first, second and third currentmeasurement pairs (I_(U), I_(V)), (I_(V), I_(W)), (I_(U), I_(W)) toproduce respective first, second and third estimates f₁, f₂, f₃ for thefrequency of the motor. This estimation may be carried out in two parts(in an analogous manner as described above for the speed estimation) sothat first separate estimates are calculated based on each current ineach current measurement pair and, then, each of said first, second andthird estimates f₁, f₂, f₃ for the frequency of the motor is calculatedbased on two estimates associated with a corresponding currentmeasurement pair (e.g., as an average). The frequency may be consideredsimply as another metric for quantifying the speed of the motor. In suchembodiments, the further analysis in elements 107 and/or 108 may becarried out using the frequency, as opposed speed of the motor in RPM.

The voting-based speed analysis unit 108 applies, in element 107, avoting logic to the first, second and third estimates for the speed ofthe motor (or the frequency) to generate a combined, final estimate andto determine whether or not the final estimate is valid (i.e.,trustworthy or dependable). The voting logic 107 may specificallycorrespond a three-channel voting system implementing a SafetyInstrumented System (SIS) voting architecture. The inputs of the votinglogic 107 (corresponding to said three channel) may be compared, by thevoting logic 107, against pre-defined criteria (e.g., an upper thresholddefining that a relative difference between an estimate and one or bothof the other two estimates should be less than 10%) and if a certainpre-defined number of the inputs (and/or optionally results of one ormore diagnostic tests) indicate a negative result (e.g., no twoestimates are within 10% from each other or all three estimates are notwithin 10% from each other), it is determined that the estimates shouldnot be trusted and thus a negative output is produced. Otherwise, apositive output may be produced.

In some embodiments, the voting logic may employ, in the voting, one ormore pre-defined criteria comprising at least an upper threshold forabsolute difference or relative difference between any two estimatesand/or an upper threshold for absolute difference or relative differencebetween an estimate and an average of the first, second and thirdestimates. For example, the voting logic using said upper threshold mayrequire that each acceptable (or valid) estimate should be within 10%from all or one or more other (acceptable) estimates. In otherembodiments, an upper threshold for absolute deviation or relativedeviation between any two or three estimates and/or an upper thresholdfor absolute deviation or relative deviation between an estimate and anaverage of the first, second and third estimates may be defined, insteador in addition. In other embodiments, the pre-defined criteria maycomprise a pre-defined upper limit for speed or frequency of the motorand/or a pre-defined lower limit for speed or frequency of the motor.

The voting logic 107 may employ a certain voting architecture, equallycalled a Safety Instrumented Function (SIF) or Safety InstrumentedSystem (SIS) architecture. Specifically, the voting logic 107 may useone of a one out of three (1oo3) voting architecture, a two out of three(2oo3) voting architecture, a three out of three (3oo3) votingarchitecture, a one out of three diagnosed (1oo3D) voting architecture,a two out of three diagnosed (2oo3D) voting architecture and a three outof three diagnosed (3oo3D) voting architecture. Two exemplary votingarchitectures, namely the 2oo3 and 1oo3D voting architectures, arediscussed in detail in relation to FIGS. 2A and 2B.

The voting logic 107 may generate as its output at least information onan estimated value of speed or frequency (i.e., a final estimategenerated based on the first, second and third estimates) and anindication (e.g., a flag) indicating whether or not the given finalestimate is valid (i.e., whether or not it can be trusted). An outputcomprising an indication indicating that the provided estimate is validand an indication indicating that that the provided estimate is invalidmay be equally called a positive output and a negative output,respectively. Depending on the state of the indication (i.e.,valid/invalid), the final estimate for the speed or frequency may beused in different ways in the safety evaluation carried out by thesafety logic 109 (or even not used at all).

In some embodiments, the output of the voting logic may, in addition,carry further information regarding the measured and/or estimated valuesand/or whether they meet the one or more pre-defined criteria.Specifically, the output of the voting logic may comprise information onone or more of:

the measured values of the first current measurement pair;

the measured values of the second current measurement pair;

the measured values of the third current measurement pair;

the first estimate for the speed or frequency of the motor;

the second estimate for the speed or frequency of the motor;

the third estimate for the speed or frequency of the motor;

information on whether the first estimate satisfies the one or morepre-defined criteria;

information on whether the second estimate satisfies the one or morepre-defined criteria;

information on whether the third estimate satisfies the one or morepre-defined criteria;

a diagnostic status of the first estimate;

a diagnostic status of the second estimate; and

a diagnostic status of the third estimate.

The diagnostic status of the first/second/third estimate mentioned abovemay comprise, for example, information on results of any diagnostictests carried out for a channel of the voting logic producing thefirst/second/third estimate, respectively.

The voting-based speed analysis unit 108 feeds the output of the votinglogic to a safety logic 109 controlling at least one safety function ofthe drive. Said at least one safety function may comprise at least theSTO safety function 110. Specifically, the safety logic 109 may beconfigured to receive the output of the voting logic, evaluate one ormore pre-defined safety criteria based at least on the output of thevoting logic and in response to at least one of the one or morepre-defined safety criteria of the drive failing to be met, trigger oneor more safety functions comprising at least the STO safety function110. The one or more pre-defined safety criteria may comprise at leastone safety criterion (e.g., a threshold value) for the estimate of thespeed of the motor and/or frequency. Said at least one safety criterionmay be evaluated only if it is indicated in the output of the votinglogic that the estimate provided therein is valid.

FIGS. 2A and 2B illustrate two exemplary alternative votingarchitectures according to embodiments which may be employed in thevoting logic of the voting-based speed analysis unit of the drive (thatis, in element 107 of FIG. 1). Namely, FIG. 2A illustrates 2oo3 votingarchitecture and FIG. 2B illustrates a 1oo3D voting architecture.

In 2oo3 voting architectures such as the one illustrated in FIG. 2A, atleast two of the inputs 201, 202, 203 must fail (i.e., indicate failure)in order for the voting logic to provide a negative output indicatingthat the estimate is not (very) dependable (and thus should probably notbe used evaluating whether or not to trigger the stopping of the motor).Thus, the 2oo3 voting architecture corresponds to a majority votingscenario.

Applying the 2oo3 voting architecture to inputs 201, 202, 203 (e.g., inelement 107 of FIG. 1) comprises the following steps. First, the votinglogic evaluates, in block 204, the first, second and third estimates forthe speed of the motor against one or more pre-defined criteria forallowed speed of the motor. Said one or more pre-defined criteria mayspecifically define an upper threshold for absolute difference (e.g.,200 RPM) or relative difference (e.g., 10%) between any two estimatesfor the speed of the motor or between any estimate and an average overthe first, second and third estimates. In other words, the voting logicmay evaluate, in block 204, whether of each of the first, second andthird estimates deviates from the other two estimates by more than isallowed by said threshold. Specifically, it may be defined that thepre-defined threshold is satisfied for a particular estimate if at leastone other estimate is within the pre-defined threshold. In otherembodiments, deviation or relative deviation may be employed, instead ofthe absolute difference or the relative difference, as described inrelation to FIG. 1.

In response to two or more of the first, second and third estimatessatisfying the one or more pre-defined criteria in block 205, the votinglogic generates, in block 206, a positive output. Specifically, saidpositive output may comprise information on an estimated value of speedor frequency (i.e., a final estimate) and an indication (e.g., a flag)indicating that the given speed or frequency estimate is valid (i.e.,that it can be trusted). The final estimated value of speed or frequencymay be calculated (or generated), by the voting logic, based on all orsome of the first, second and third estimates or specifically based onone or more of the first, second and third estimates satisfying the oneor more pre-defined criteria (if any exist). The estimated value may becalculated, for example, as an average, a median, a minimum or a maximumover any aforementioned set of one or more estimates.

Conversely, in response to two or more of the first, second and thirdestimates failing to satisfy the one or more pre-defined criteria (orequally in response to only zero or one of the first, second and thirdestimates satisfying the one or more pre-defined criteria), the votinglogic generates, in block 207, a negative output. Specifically, saidnegative output may comprise information on an estimated value of speedor frequency and an indication (e.g., a flag) indicating that the givenspeed or frequency estimate is invalid (i.e., that it cannot betrusted). In this case, the estimated value may be a value satisfyingthe one or more pre-defined criteria or it may be calculated, forexample, as an average, a median, a minimum or a maximum over allestimates.

Embodiments using 1oo3 and 3oo3 voting architectures may operate in afully analogous manner compared to the 2oo3 voting architecture asdiscussed in relation to FIG. 2A. The only difference may be, obviously,that the number of estimates that need to satisfy the one or morepre-defined criteria for generating a positive output is one or more(1oo3) or three (3oo3).

In 1oo3D voting architectures such as the one illustrated in FIG. 2B,all three of the inputs 211, 212, 213 must fail (or indicate failure) inorder for the voting logic to generate a negative output. Furthermore,1oo3D voting architecture may utilize (external) diagnostic coverage(i.e., one or more diagnostic tests performed periodically) to improvethe detection of dangerous faults (as indicated by the “D” for diagnosedin “1oo3D”).

Applying the 1oo3D voting architecture to inputs 211, 212, 213 (e.g., inelement 107 of FIG. 1) comprises the following steps. First, the votinglogic evaluates, in block 214, the first, second and third estimates forthe speed of the motor separately against one or more pre-definedcriteria (as described in relation to FIG. 1 and/or FIG. 2A). Then, thevoting logic performs, in block 215, one or more diagnostic tests.Specifically, the voting logic may perform one or more diagnostic testsfor each of three channels of the voting logic corresponding to thefirst, second and third estimates. For example, the one or morediagnostic tests may comprise one or more chip input voltage testsand/or one or more memory tests.

In response to all of the first, second and third estimates failing tosatisfy the one or more pre-defined criteria, the voting logicgenerates, in block 219, a negative output. In response to one or moreof the first, second and third estimates satisfying the one or morepre-defined criteria in block 216, it is further determined whether theone or more diagnostic tests indicated correct operation for one or morechannels of the voting logic corresponding to said one or more of thefirst, second and third estimates satisfying the one or more pre-definedcriteria in block 217. The voting logic may effectively disable one ormore channels of the voting logic based on the results of the one ormore diagnostic tests in block 217. Thus, in response to none of thefirst, second and third estimates satisfying the one or more pre-definedcriteria while also passing (or to be precise, the corresponding channelpassing) the one or more diagnostic tests in block 218, the voting logicalso generates, in block 219, a negative output. Only in response to oneor more of the first, second and third estimates satisfying the one ormore pre-defined criteria and one or more corresponding channels passingthe one or more diagnostic tests in block 218, does the voting logicgenerate, in block 218, a positive output. The negative and positiveoutput may be defined as described in relation to FIGS. 1 and 2A thoughin some embodiments said negative and positive outputs may also compriseinformation on the results of the one or more diagnostic tests.

In some embodiments, the checks in blocks 216, 217 may be carried out ina different order.

It should be noted that the one or more diagnostic tests may be carriedout periodically with a different period compared to the period ofmeasuring the three phase currents. Specifically, while the measuring ofthe three phase currents may be effectively a continuous process (or aprocess associated with a very small period), the one or more diagnostictests may be carried out less frequently. Despite of this, the operationof the voting logic may also be continuous so that the performing theone or more diagnostic tests in block 215 may comprise merely retrievingthe most recent values of the one or more diagnostic tests from amemory.

Embodiments using 2oo3D and 3oo3D voting architectures may operate in afully analogous manner compared to the 1oo3D voting architecture asdiscussed in relation to FIG. 2B. The only difference may be, obviously,that the number of estimates that need to satisfy the one or morepre-defined criteria and pass the one or more diagnostic test forgenerating a positive output is two or more (2oo3D) or three (3oo3D).

While the inputs 201, 202, 203 (n₁, n₂ and n₃) correspond in FIGS. 2Aand 2B to estimated speeds of the motor, in other embodiments estimatedfrequencies of the motor (f₁, f₂ and f₃) may be used, instead, in thevoting, similar to as described in relation to FIG. 1. Correspondingly,in such embodiments, the one or more pre-defined criteria may be definedfor allowed frequency of the motor and the output of the voting logicmay comprise information on an estimated value of frequency and anindication (e.g., a flag) indicating that the given frequency estimateis invalid (i.e., that it cannot be trusted). In some embodiments, theoutput quantity (i.e., speed or frequency) may be different from thequantity (i.e., frequency or speed, respectively) used in the voting.

FIG. 3 illustrates a process according to embodiments carried out by asafety logic. Specifically, the illustrated process may be carried outby the safety logic in response to receiving the output of the votinglogic. The safety logic in question may be the safety logic 109 of FIG.1.

Referring to FIG. 3, the safety logic receives, in block 301, an outputof a voting logic of the voting-based speed analysis unit. Uponreception, the voting logic evaluates, in block 302, one or morepre-defined safety criteria based at least on the output of the votinglogic. In response to at least one of the one or more pre-defined safetycriteria of the frequency converter failing to be met in block 303, thesafety logic triggers, in block 304, at least one safety function. Saidat least one safety function may comprise at least a safe torque off,STO, safety function.

In some embodiments, the one or more pre-defined safety criteria maycomprise a pre-defined threshold (or upper and lower thresholds) for aspeed or a frequency of a motor controlled by the frequency converter.Here, it is assumed also that the output of the voting logic received inblock 301 comprises a final estimate for said speed or frequency of themotor and an indication whether or not said final estimate is validaccording to the voting logic. In such embodiments, the evaluating ofthe one or more pre-defined safety criteria in block 302 comprisesevaluating the final estimate against the pre-defined threshold but onlyin response to the indication indicating that the final estimate isvalid. If the final estimate is indicated not to be valid, it may beignored or used in other (less vital) processes.

The blocks, related functions, and information exchanges described aboveby means of FIGS. 1, 2A, 2B and 3 are in no absolute chronologicalorder, and some of them may be performed simultaneously or in an orderdiffering from the given one. Other functions can also be executedbetween them or within them, and other information may be sent, and/orother rules applied. Some of the blocks or part of the blocks or one ormore pieces of information can also be left out or replaced by acorresponding block or part of the block or one or more pieces ofinformation.

FIG. 4 provides a voting-based speed analysis unit 401 according to someembodiments. Specifically, FIG. 4 may illustrate a voting-based speedanalysis unit configured to carry out at least the functions describedabove in connection with estimating speed or frequency of the motor anddetermining validity of the estimation. The voting-based speed analysisunit 401 may be a voting-based speed analysis unit 108 of FIG. 1. Thevoting-based speed analysis unit 401 may comprise one or more controlcircuitry 420, such as at least one processor, and at least one memory430, including one or more algorithms 431, such as a computer programcode (software) wherein the at least one memory and the computer programcode (software) are configured, with the at least one processor, tocause the voting-based speed analysis unit to carry out any one of theexemplified functionalities of the voting-based speed analysis unitdescribed above. The at least one processor may be or may comprise, forexample, a general-purpose digital signal processor (DSP). Said at leastone memory 430 may also comprise at least one database 432.

Referring to FIG. 4, the one or more control circuitry 420 of thevoting-based speed analysis unit 401 comprise at least measurementcircuitry 421 which is configured to perform measuring of the threephase currents and determining the associated frequencies and speed ofthe motor. To this end, the measurement circuitry 421 is configured tocarry out functionalities described above by means of any of elements101 to 106 of FIG. 1 using one or more individual circuitries. The oneor more control circuitry 420 of the voting-based speed analysis unit401 further comprises voting circuitry 422 for implementing a votingarchitecture. The voting circuitry 422 is configured to carry outfunctionalities described above by means of any of block 107 of FIG. 1,FIG. 2A and/or FIG. 2B using one or more individual circuitries. It isalso feasible to use specific integrated circuits, such as ASIC(Application Specific Integrated Circuit) or other components anddevices for implementing the functionalities in accordance withdifferent embodiments.

Referring to FIG. 4, the voting-based speed analysis unit 401 mayfurther comprise different interfaces 410 such as one or morecommunication interfaces comprising hardware and/or software forrealizing communication connectivity according to one or morecommunication protocols. Specifically, the one or more communicationinterfaces 410 may comprise, for example, an interface providing aconnection to a safety logic. The one or more communication interfaces410 may comprise standard well-known components such as an amplifier,filter, frequency-converter, (de)modulator, and encoder/decodercircuitries, controlled by the corresponding controlling units, and oneor more antennas.

FIG. 5 provides a safety logic 501 according to some embodiments.Specifically, FIG. 5 may illustrate a safety logic configured to carryout at least the functions described above in connection with receivingan output of a voting logic, evaluating one or more safety criteriabased at least on said output and triggering a STO and/or other safetyfunction(s) if safety is detected to be compromised. The safety logic501 may be a safety logic 109 of FIG. 1. The safety logic 501 maycomprise one or more control circuitry 520, such as at least oneprocessor, and at least one memory 530, including one or more algorithms531, such as a computer program code (software) wherein the at least onememory and the computer program code (software) are configured, with theat least one processor, to cause the safety logic to carry out any oneof the exemplified functionalities of the safety logic described above.The at least one processor may be or may comprise, for example, ageneral-purpose digital signal processor (DSP). Said at least one memory530 may also comprise at least one database 532.

Referring to FIG. 5, the one or more control circuitry 520 of safetylogic 501 comprise at least safety analysis circuitry 521 which isconfigured to perform safety analysis regarding the motor and to controlone or more safety functions accordingly. To this end, the safety logic521 is configured to carry out functionalities described above by meansof element 109 of FIG. 1 and FIG. 3 using one or more individualcircuitries. It is also feasible to use specific integrated circuits,such as ASIC (Application Specific Integrated Circuit) or othercomponents and devices for implementing the functionalities inaccordance with different embodiments.

Referring to FIG. 5, the safety logic 501 may further comprise differentinterfaces 510 such as one or more communication interfaces comprisinghardware and/or software for realizing communication connectivityaccording to one or more communication protocols. Specifically, the oneor more communication interfaces 510 may comprise, for example, aninterface providing a connection to one or more safety functionscomprising at least a STO. The one or more communication interfaces 510may comprise standard well-known components such as an amplifier,filter, frequency-converter, (de)modulator, and encoder/decodercircuitries, controlled by the corresponding controlling units, and oneor more antennas.

Referring to FIGS. 4 and 5, the memories 430, 530 may be implementedusing any suitable data storage technology, such as semiconductor basedmemory devices, flash memory, magnetic memory devices and systems,optical memory devices and systems, fixed memory and removable memory.

As used in this application, the term ‘circuitry’ may refer to one ormore or all of the following: (a) hardware-only circuit implementations,such as implementations in only analog and/or digital circuitry, and (b)combinations of hardware circuits and software (and/or firmware), suchas (as applicable): (i) a combination of analog and/or digital hardwarecircuit(s) with software/firmware and (ii) any portions of hardwareprocessor(s) with software, including digital signal processor(s),software, and memory(ies) that work together to cause an apparatus, suchas a terminal device or an access node, to perform various functions,and (c) hardware circuit(s) and processor(s), such as amicroprocessor(s) or a portion of a microprocessor(s), that requiressoftware (e.g. firmware) for operation, but the software may not bepresent when it is not needed for operation. This definition of‘circuitry’ applies to all uses of this term in this application,including any claims. As a further example, as used in this application,the term ‘circuitry’ also covers an implementation of merely a hardwarecircuit or processor (or multiple processors) or a portion of a hardwarecircuit or processor and its (or their) accompanying software and/orfirmware. The term ‘circuitry’ may also cover the term ‘logic’ (as usedin, e.g., ‘voting logic’).

In an embodiment, at least some of the processes described in connectionwith FIGS. 1, 2A, 2B and 3 may be carried out by an apparatus comprisingcorresponding means for carrying out at least some of the describedprocesses. Some example means for carrying out the processes may includeat least one of the following: detector, processor (including dual-coreand multiple-core processors), digital signal processor, controller,receiver, transmitter, encoder, decoder, memory, RAM, ROM, software,firmware, display, user interface, display circuitry, user interfacecircuitry, user interface software, display software, circuit, filter(low-pass, high-pass, bandpass and/or bandstop), sensor, circuitry,inverter, capacitor, inductor, resistor, operational amplifier, diodeand transistor. In an embodiment, the at least one processor, thememory, and the computer program code form processing means or comprisesone or more computer program code portions for carrying out one or moreoperations according to any one of the embodiments of FIGS. 1, 2A, 2Band 3 or operations thereof. In some embodiments, at least some of theprocesses may be implemented using discrete components.

Embodiments as described may also be carried out, fully or at least inpart, in the form of a computer process defined by a computer program orportions thereof. Embodiments of the methods described in connectionwith FIGS. 1, 2A, 2B and 3 may be carried out by executing at least oneportion of a computer program comprising corresponding instructions. Thecomputer program may be provided as a computer readable mediumcomprising program instructions stored thereon or as a non-transitorycomputer readable medium comprising program instructions stored thereon.The computer program may be in source code form, object code form, or insome intermediate form, and it may be stored in some sort of carrier,which may be any entity or device capable of carrying the program. Forexample, the computer program may be stored on a computer programdistribution medium readable by a computer or a processor. The computerprogram medium may be, for example but not limited to, a record medium,computer memory, read-only memory, electrical carrier signal,telecommunications signal, and software distribution package, forexample. The computer program medium may be a non-transitory medium.Coding of software for carrying out the embodiments as shown anddescribed is well within the scope of a person of ordinary skill in theart.

The embodiments may be implemented in existing systems, such as ininverters and/or surveillance systems of drives, or discrete elementsand devices may be used in a centralized or decentralized manner.Existing devices, such as inverters, typically comprise a processor anda memory which may be utilized in implementing the functionality of theembodiments. Thus, the changes and assemblies required by theimplementation of the embodiments may, at least partly, be taken care ofby software routines, which, in turn, may be implemented as added orupdated software routines (being defined as described in the previousparagraph).

Even though the embodiments have been described above with reference toexamples according to the accompanying drawings, it is clear that theembodiments are not restricted thereto but can be modified in severalways within the scope of the appended claims. Therefore, all words andexpressions should be interpreted broadly and they are intended toillustrate, not to restrict, the embodiment. It will be obvious to aperson skilled in the art that, as technology advances, the inventiveconcept can be implemented in various ways. Further, it is clear to aperson skilled in the art that the described embodiments may, but arenot required to, be combined with other embodiments in various ways.

The invention claimed is:
 1. A method for evaluating a speed or afrequency of a motor controlled by a frequency converter, the methodcomprising: measuring first, second and third phase currents ofthree-phase electric power fed from the frequency converter to themotor; forming first, second and third current measurement pairs,wherein the first current measurement pair comprises the first phasecurrent and the second phase current, the second current measurementpair comprises the second phase current and the third phase current andthe third current measurement pair comprises the third phase current andthe first phase current; estimating the speed or frequency of the motorseparately based on each pair of the first, second and third currentmeasurement pairs to produce respective first, second and thirdestimates for the speed or frequency of the motor, applying a votinglogic to the first, second and third estimates for the speed orfrequency of the motor, wherein the voting logic employs, in the voting,one or more pre-defined criteria and is based on one of the following: aone out of three, 1oo3, voting architecture, a two out of three, 2oo3,voting architecture, a three out of three, 3oo3, voting architecture, aone out of three diagnosed, 1oo3D, voting architecture, a two out ofthree diagnosed, 2oo3D, voting architecture and a three out of threediagnosed, 3oo3D, voting architecture; and feeding an output of thevoting logic to a safety logic controlling at least one safety functionof the frequency converter, wherein the output of the voting logiccomprises a final estimate for the speed or frequency of the motor andan indication whether or not said final estimate is valid according tothe voting logic.
 2. The method of claim 1, wherein the one or morepre-defined criteria comprise at least an upper threshold for absolutedifference or relative difference, between any two estimates or betweenan estimate and an average of the first, second and third estimates oran upper threshold for absolute deviation or relative deviation betweenany two or three estimates or between an estimate and an average of thefirst, second and third estimates.
 3. The method of claim 2, whereinsaid at least one safety function comprises at least a safe torque off,STO, safety function.
 4. The method according to claim 3, wherein theestimating of the speed of the motor separately based on each of thefirst current measurement pair, the second current measurement pair andthe third current measurement pair comprises performing, for eachcurrent measurement pair, the following: estimating a frequency of thethree-phase electric power fed to the motor separately based on each oftwo phase currents in said current measurement pair; calculating twopreliminary estimates for the speed of the motor separately based on theestimated frequencies; and calculating an estimate for the speed of themotor as an average over the two preliminary estimates for the speed ofthe motor, wherein the estimating of the frequency of the motorseparately based on each of the first current measurement pair, thesecond current measurement pair and the third current measurement paircomprises performing, for each current measurement pair, the following:estimating a frequency of the three-phase electric power fed to themotor by averaging frequencies calculated based on two phase currents insaid current measurement pair.
 5. The method according to claim 2,wherein the estimating of the speed of the motor separately based oneach of the first current measurement pair, the second currentmeasurement pair and the third current measurement pair comprisesperforming, for each current measurement pair, the following: estimatinga frequency of the three-phase electric power fed to the motorseparately based on each of two phase currents in said currentmeasurement pair; calculating two preliminary estimates for the speed ofthe motor separately based on the estimated frequencies; and calculatingan estimate for the speed of the motor as an average over the twopreliminary estimates for the speed of the motor, wherein the estimatingof the frequency of the motor separately based on each of the firstcurrent measurement pair, the second current measurement pair and thethird current measurement pair comprises performing, for each currentmeasurement pair, the following: estimating a frequency of thethree-phase electric power fed to the motor by averaging frequenciescalculated based on two phase currents in said current measurement pair.6. The method of claim 1, wherein said at least one safety functioncomprises at least a safe torque off, STO, safety function.
 7. Themethod according to claim 6, wherein the estimating of the speed of themotor separately based on each of the first current measurement pair,the second current measurement pair and the third current measurementpair comprises performing, for each current measurement pair, thefollowing: estimating a frequency of the three-phase electric power fedto the motor separately based on each of two phase currents in saidcurrent measurement pair; calculating two preliminary estimates for thespeed of the motor separately based on the estimated frequencies; andcalculating an estimate for the speed of the motor as an average overthe two preliminary estimates for the speed of the motor, wherein theestimating of the frequency of the motor separately based on each of thefirst current measurement pair, the second current measurement pair andthe third current measurement pair comprises performing, for eachcurrent measurement pair, the following: estimating a frequency of thethree-phase electric power fed to the motor by averaging frequenciescalculated based on two phase currents in said current measurement pair.8. The method according to claim 1, wherein the estimating of the speedof the motor separately based on each of the first current measurementpair, the second current measurement pair and the third currentmeasurement pair comprises performing, for each current measurementpair, the following: estimating a frequency of the three-phase electricpower fed to the motor separately based on each of two phase currents insaid current measurement pair; calculating two preliminary estimates forthe speed of the motor separately based on the estimated frequencies;and calculating an estimate for the speed of the motor as an averageover the two preliminary estimates for the speed of the motor, whereinthe estimating of the frequency of the motor separately based on each ofthe first current measurement pair, the second current measurement pairand the third current measurement pair comprises performing, for eachcurrent measurement pair, the following: estimating a frequency of thethree-phase electric power fed to the motor by averaging frequenciescalculated based on two phase currents in said current measurement pair.9. The method according to claim 1, wherein the voting logic uses the2oo3 voting architecture, the applying of the voting logic comprising:evaluating the first, second and third estimates for the speed orfrequency of the motor separately against the one or more pre-definedcriteria; in response to two or more of the first, second and thirdestimates failing to satisfy the one or more pre-defined criteria,generating a negative output comprising the final estimate for the speedor frequency of the motor and an indication indicating that the finalestimate is not valid; and in response to two or more of the first,second and third estimates satisfying the one or more pre-definedcriteria, generating a positive output comprising the final estimate forthe speed or frequency of the motor and an indication indicating thatthe final estimate is valid.
 10. The method according to claim 1,wherein the voting logic uses the 1oo3D voting architecture, theapplying of the voting logic comprising: evaluating the first, secondand third estimates for the speed or frequency of the motor separatelyagainst the one or more pre-defined criteria; performing one or morediagnostic tests for each of three channels of the voting logiccorresponding to the first, second and third estimates; in response tonone of the first, second and third estimates satisfying the one or morepre-defined criteria while also passing the one or more diagnostictests, generating a negative output comprising the final estimate forthe speed or frequency of the motor and an indication indicating thatthe estimate is not valid; and in response to one or more of the first,second and third estimates satisfying the one or more pre-definedcriteria while also passing the one or more diagnostic tests, generatinga positive output comprising the final estimate for the speed orfrequency of the motor and an indication indicating that the estimate isvalid.
 11. The method of claim 10, wherein said one or more diagnostictests comprise one or more chip input voltage tests and/or one or morememory tests.
 12. The method according to claim 1, wherein the finalestimate for the speed or frequency of the motor is generated, by thevoting logic, based on all of the first, second and third estimates orbased on one or more of the first, second and third estimates satisfyingthe one or more pre-defined criteria.
 13. The method according to claim1, wherein the final estimate is calculated as an average, a median, aminimum or a maximum over a set of one or more estimates.
 14. The methodaccording to claim 1, wherein each output of the voting logic comprisesinformation on one or more of: the measured values of the first currentmeasurement pair; the measured values of the second current measurementpair; the measured values of the third current measurement pair; thefirst estimate for the speed or frequency of the motor; the secondestimate for the speed or frequency of the motor; the third estimate forthe speed or frequency of the motor; information on whether the firstestimate satisfies the one or more pre-defined criteria; information onwhether the second estimate satisfies the one or more pre-definedcriteria; information on whether the third estimate satisfies the one ormore pre-defined criteria; a diagnostic status of the first estimate; adiagnostic status of the second estimate; and a diagnostic status of thethird estimate.
 15. An apparatus comprising: at least one processor; andat least one memory including computer program code, said at least onememory and computer program code being configured, with said at leastone processor, to cause measuring first, second and third phase currentsof three-phase electric power fed from a frequency converter to a motor;forming first, second and third current measurement pairs, wherein thefirst current measurement pair comprises the first phase current and thesecond phase current, the second current measurement pair comprises thesecond phase current and the third phase current and the third currentmeasurement pair comprises the third phase current and the first phasecurrent; estimating a speed or a frequency of the motor separately basedon each pair of the first, second and third current measurement pairs toproduce respective first, second and third estimates for the speed orfrequency of the motor, applying a voting logic to the first, second andthird estimates for the speed or frequency of the motor, wherein thevoting logic employs, in the voting, one or more pre-defined criteriaand is based on one of the following: a one out of three, 1oo3, votingarchitecture, a two out of three, 2oo3, voting architecture, a three outof three, 3oo3, voting architecture, a one out of three diagnosed,1oo3D, voting architecture, a two out of three diagnosed, 2oo3D, votingarchitecture; and feeding an output of the voting logic to a safetylogic controlling at least one safety function of the frequencyconverter, wherein the output of the voting logic comprises a finalestimate for the speed or frequency of the motor and an indicationwhether or not said final estimate is valid according to the votinglogic.
 16. A non-transitory computer readable media having storedthereon instructions that, when executed by a computing device, causethe computing device to perform an evaluation of a speed or a frequencyof a motor controlled by a frequency converter, comprising: measuringfirst, second and third phase currents of three-phase electric power fedfrom the frequency converter to the motor; forming first, second andthird current measurement pairs, wherein the first current measurementpair comprises the first phase current and the second phase current, thesecond current measurement pair comprises the second phase current andthe third phase current and the third current measurement pair comprisesthe third phase current and the first phase current; estimating thespeed or frequency of the motor separately based on each pair of thefirst, second and third current measurement pairs to produce respectivefirst, second and third estimates for the speed or frequency of themotor, applying a voting logic to the first, second and third estimatesfor the speed or frequency of the motor, wherein the voting logicemploys, in the voting, one or more pre-defined criteria and is based onone of the following: a one out of three, 1oo3, voting architecture, atwo out of three, 2oo3, voting architecture, a three out of three, 3oo3,voting architecture, a one out of three diagnosed, 1oo3D, votingarchitecture, a two out of three diagnosed, 2oo3D, voting architecture;and feeding an output of the voting logic to a safety logic controllingat least one safety function of the frequency converter, wherein theoutput of the voting logic comprises a final estimate for the speed orfrequency of the motor and an indication whether or not said finalestimate is valid according to the voting logic.
 17. A frequencyconverter comprising: a voting-based speed analysis unit comprising atleast one processor; and at least one memory including computer programcode, said at least one memory and computer program code beingconfigured, with said at least one processor, to cause; measuring first,second and third phase currents of three-phase electric power fed fromthe frequency converter to the motor; forming first, second and thirdcurrent measurement pairs, wherein the first current measurement paircomprises the first phase current and the second phase current, thesecond current measurement pair comprises the second phase current andthe third phase current and the third current measurement pair comprisesthe third phase current and the first phase current; estimating thespeed or frequency of the motor separately based on each pair of thefirst, second and third current measurement pairs to produce respectivefirst, second and third estimates for the speed or frequency of themotor, applying a voting logic to the first, second and third estimatesfor the speed or frequency of the motor, wherein the voting logicemploys, in the voting, one or more pre-defined criteria and is based onone of the following: a one out of three, 1oo3, voting architecture, atwo out of three, 2oo3, voting architecture, a three out of three, 3oo3,voting architecture, a one out of three diagnosed, 1oo3D, votingarchitecture, a two out of three diagnosed, 2oo3D, voting architecture;and feeding an output of the voting logic to a safety logic controllingat least one safety function of the frequency converter, wherein theoutput of the voting logic comprises a final estimate for the speed orfrequency of the motor and an indication whether or not said finalestimate is valid according to the voting logic; and a safety logiccomprising at least one processor; and at least one memory includingcomputer program code, said at least one memory and computer programcode being configured, with said at least one processor, to cause:receiving an output of a voting logic of the voting-based speed analysisunit; evaluating one or more pre-defined safety criteria based at leastone the output of the voting logic; and in response to at least one ofthe one or more pre-defined safety criteria of the frequency converterfailing to be met, triggering at least a safe torque off, STO, safetyfunction.
 18. The frequency converter of claim 17, further comprising:one or more safety functions comprising at least the STO safetyfunction.
 19. The frequency converter of claim 18, wherein the one ormore pre-defined safety criteria comprise a pre-defined threshold for aspeed or a frequency of a motor controlled by the frequency converterand the output of the voting logic comprises a final estimate for saidspeed or frequency of the motor and an indication whether or not saidfinal estimate is valid according to the voting logic, the evaluating ofthe one or more pre-defined safety criteria comprising: in response tothe indication indicating that the final estimate is valid, evaluatingthe final estimate against the pre-defined threshold.
 20. The frequencyconverter of claim 17, wherein the one or more pre-defined safetycriteria comprise a pre-defined threshold for a speed or a frequency ofa motor controlled by the frequency converter and the output of thevoting logic comprises a final estimate for said speed or frequency ofthe motor and an indication whether or not said final estimate is validaccording to the voting logic, the evaluating of the one or morepre-defined safety criteria comprising: in response to the indicationindicating that the final estimate is valid, evaluating the finalestimate against the pre-defined threshold.